1. Field of the Invention
The present invention relates to methods and apparatuses for determining a rate of deterioration of three way reducing and oxidising catalysts in a system having double air-fuel ratio sensors upstream and downstream of the catalysts in an exhaust gas passage.
2. Description of the Related Art
In a known air-fuel ratio control system, an air-fuel ratio sensor is disposed in the exhaust passage near a combustion chamber, i.e., near the concentration portion of the exhaust manifold. In this system, however, accurate air-fuel ratio control is disturbed because of the dispersion of the output characteristics of the air-fuel ratio sensor.
To compensate for the above dispersion, and to compensate for the dispersion and the aged deterioration of parts used for fuel injection, a so-called double air-fuel sensor system that controls the air-fuel ratio based on an output of a second air-fuel ratio sensor disposed in the downstream side of the catalysts in addition to an output of the first air-fuel ratio sensor disposed in the upstream side of the catalysts is suggested (See Japanese Unexamined Patent Application (Kokai) No. 61-286550).
In this system, the second air-fuel ratio sensor has a poorer response speed but smaller dispersion than the first air-fuel ratio sensor for the following reasons.
(1) The second air-fuel ratio sensor receives less thermal influence of the exhaust gas, because its temperature is low at the downstream side of the catalyst.
(2) The second air-fuel sensor is exposed to less poisoning, because various kinds of polutants are trapped in the catalyst.
(3) Exhaust gas attains uniformity and a state of equilibrium at the downstreams side of the catalyst.
The double air-fuel sensor system, therefore, can compensate for the dispersion of the first air-fuel ratio sensor by using the second air-fuel ratio sensor. That is, in the single air-fuel ratio sensor system, HC, CO and NO.sub.x emissions increase after the first air-fuel ratio sensor deteriorates, but in the double air-fuel ratio sensor system, HC, CO and NO.sub.x emissions do not increase even after the first air-fuel ratio sensor deteriorates as long as the second air-fuel ratio sensor maintains stability.
In the double air-fuel ratio sensor system, however, after the catalyst deteriorates, HC, CO and NO.sub.x emissions increase and the output of the second air-fuel ratio sensor fluctuates. That is, an amplitude of an output voltage of the second air-fuel ratio sensor increases and its period becomes shorter.
The detection of the deterioration of the catalyst, therefore, is important to obtain normal emission, and the methods to determine whether or not the catalyst has deteriorated for example based on the ratio of the inverting period of the output voltage of the second air-fuel ratio sensor to that of the first air-fuel ratio sensor is suggested (See Japanese Unexamined Patent Application (Kokai) No. 61-286550).
With the above method, however, it is possible to distinguish a deterioration of the catalyst, for example the rejection ratio of HC becomes less than 80%, from the normal condition, that is, the rejection ratio of HC remains more than 80%. But it is not possible to distinguish an extremely abnormal deterioration that requires the replacement of the catalyst, for example the rejection ratio of HC becomes less than 50%, from a normal aged deterioration that remains enough rejection ratio, that is, 80%.
As a result, HC, CO and NO.sub.x emissions are increased if the replacement of the catalyst is delayed.